Conventionally, in the field of laser beam scanning apparatuses of this type, for example as disclosed in U. S. Pat. Nos. 5,801,869 and 5,408,095, a technique has been proposed whereby two mirrors are combined together to form a scanning optical system so as to realize a laser beam scanning apparatus that offers stable image-formation performance against wavelength fluctuations in the light emitted from a light source.
In this construction, however, the mirrors are each so designed to have a reflecting surface that is rotation-symmetric about an axis. This makes it impossible to achieve satisfactory correction of aberrations. For example, in the construction disclosed in U.S. Pat. No. 5,408,095, the two mirrors constituting the scanning optical system have their reflecting surfaces formed as an axisymmetric aspherical surface and a cylindrical surface, respectively. This makes it difficult to achieve proper correction of f.theta. characteristics and curvature of field.
On the other hand, in the construction disclosed in U.S. Pat. No. 5,801,869, the two mirrors constituting the scanning optical system have their reflecting surfaces formed as a barrel-shaped toric surface and a toric surface, respectively, or alternatively an axisymmetric aspherical surface and a barrel-shaped toric surface, respectively. This allows proper correction of f.theta. characteristics and curvature of field, but makes it difficult to achieve proper correction of unevenness of the magnification in the sub-scanning-direction and groove-shaped aberration.